A tetrad of Blood Moons sounds very dramatic. It could even be the title of a fantasy novel. Indeed some of the things I've seen on the internet do read that way. But what is a “Blood Moon”? What is a lunar tetrad? And is it as apocalyptic as it sounds? Let's see.

“Blood Moon”
Blood Moon isn't an astronomical term, but seems to have been introduced by media prophets and journalists. Even some astronomers are now using it, probably because it gets more attention than total lunar eclipse, which is what a “Blood Moon” is. And despite being overly sensational, the image of a blood-soaked moon is derived from a genuine phenomenon associated with lunar eclipses.

When the Moon, Earth and Sun are are all lined up, the Earth casts a shadow on the Moon. As the Moon enters the outer part of the shadow, the penumbra, the effects are scarcely noticeable, because the Earth is blocking only some of the sunlight. The umbra, on the other hand, is a deep shadow. When the Moon is there, there's no direct sunlight, and you'd expect it to be completely dark. But it is faintly illuminated by indirect sunlight.

If Earth had no atmosphere, there wouldn't be any indirect light to reflect off the Moon during totality. However some sunlight is refracted (bent) by the atmosphere, and travels through it into the umbra.

Sunlight contains all the colors of the spectrum, but the particles of the air scatter the blue-green part of the spectrum, letting the redder colors pass through. The greater the distance the light travels through the atmosphere, the greater is this filtering effect. For example, when we're looking straight up during the day, the sky is blue because the light has traveled only a short distance through the atmosphere. At the opposite extreme, there are red skies at dawn and sunset when sunlight travels through much more atmosphere.

During a total eclipse, the fainter and redder light is what reaches the Moon to be reflected back to us. As far back as the eighth century, the Anglo-Saxon Chronicle described an eclipsed Moon as "sprinkled with blood".

Varied color
Since it's the atmosphere that allows the eclipsed Moon to be visible and colored, the state of the atmosphere affects the the Moon's appearance. The color and brightness depend on the dust particles, clouds, pollutants, etc. in the air. Different particles filter the sunlight in slightly different ways. A totally eclipsed Moon isn't always red. It can look yellow, orange or brown.

In addition, volcanic ash in the atmosphere produces quite dark eclipses, because it decreases the amount of light that penetrates the atmosphere. Although volcanic ash produces brilliant sunsets, it makes for quite dark lunar eclipses. The Danjon scale shows some of the variations in eclipse color. A very dark lunar eclipse in 1881 followed the famous eruption of Krakatoa.

Lunar tetrad
A tetrad is simply a group or series of four things, as is a quartet.

We can get between one and five lunar eclipses in a year, but only about a third of them are total. Some years there are no total eclipses. On the other hand, occasionally there are four total lunar eclipses in a row, spaced about six months apart over two years. This is what we mean by a tetrad. The 21st century will have eight tetrads. The first of them occurred in the years 2003-2004 and the second in 2014-2015.

Eight is the maximum number of tetrads that can occur in a hundred years, so the 21st century is rich in them. But Italian astronomer Giovanni Schiaparelli (1835-1910) discovered something interesting about the frequency of tetrads. Although in a 300-year period there are around seventeen lunar eclipse tetrads, such periods alternate with three hundred years with no tetrads at all. For example, from 1909-2156 there are sixteen tetrads, but there were none from 1582-1908, and will be none from 2157-2448.

The end of the world?
Besides calling the total lunar eclipses “Blood Moons”, some prophets of doom were claiming the 2014-2015 tetrad was significant because each of the four eclipses coincides with an important Jewish festival, either Passover or Sukkot.

It's worth noting that the Jewish calendar is based on the Moon, and both Passover and Sukkot always take place at the time of the full Moon. So do total lunar eclipses. In a tetrad, the eclipses are about six months apart and so are the two festivals. If the first eclipse of a tetrad falls on Passover, the other eclipses will also coincide with festivals. The coincidence of these festivals with the lunar eclipses is rare, but not as astonishing as it might sound. And of course, following 2014-2015, the world is still here.